FILE - In this Dec. 23, 2013, file photo, former San Francisco 49ers wide receiver Dwight Clark is honored at halftime of an NFL football game between the 49ers and the Atlanta Falcons in San Francisco. Clark stands near the spot where he made a catch so famous it is referred to as "The Catch." Clark says he has Lou Gehrig’s disease and suspects playing football might have caused the illness. Clark announced Sunday, March 19, 2017. on Twitter that he has amyotrophic lateral sclerosis, a disease that attacks cells that control muscles. Clark linked to a post on his personal blog detailing his ALS diagnosis, but the site crashed Sunday night, apparently from an overflow of traffic. (AP Photo/Tony Avelar, File)

SAN FRANCISCO (AP) — Dwight Clark revealed Sunday that he has Lou Gehrig's disease and suspects playing football might have caused the illness.

Clark announced on Twitter that he has amyotrophic lateral sclerosis, a disease that attacks cells that control muscles. The former San Francisco 49ers wide receiver linked to a post on his personal blog detailing his ALS diagnosis, but the site crashed Sunday night, apparently from an overflow of traffic.

"I've been asked if playing football caused this," Clark said in the post. "I don't know for sure. But I certainly suspect it did."

The 60-year-old Clark wrote that he began experiencing symptoms in September 2015. He's lost significant strength in his left hand and also has weakness in his right hand, midsection, lower back and right leg.

"I can't run, play golf or walk any distances," he said. "Picking up anything over 30 pounds is a chore. The one piece of good news is that the disease seems to be progressing more slowly than in some patients."

Clark won two Super Bowls with the 49ers during a nine-year career that ended in 1987. He memorably pulled down the winning touchdown pass from Joe Montana in the 1981 NFC Championship Game against the Dallas Cowboys, a play remembered simply as "The Catch."

Clark, whose No. 87 has been retired by the 49ers, also encouraged the NFL and the players' association to work together in making football safer.

San Francisco CEO Jed York said in a statement he was "deeply saddened" by Clark's diagnosis.

"Many know Dwight as an iconic figure in 49ers lore, whose accomplishments on the field brought joy to fans around the world," York said. "Our organization is fortunate to know him more intimately as a wonderful man who has given so much of himself as an ambassador to the entire Bay Area. We will stand alongside Dwight and his family as they wage this battle."

After his playing career ended, Clark served as general manager of the 49ers and Cleveland Browns.